Writing a DB

Objective: write fast and simple database program. Interaction with external applications through socket.
What's the best way to organise memory usage and file storage? I've come up with reading file and decomposing it into array elements. But there can be problems as file size limit and memory usage restrictions.
Any ideas?

If the point is to write it yourself, NoSQL will be much easier to implement than anything else, simply because it barely does anything.
If the point is to have a working database, use Postgres.